This invention relates to light emitting semiconductor devices, such as lasers and LEDs, and more particularly to the confinement of current flow in these devices.
One of the earliest structures for confining current to flow in a relatively narrow channel through the active region of a light emitting device was the stripe geometry contact first proposed for semiconductor lasers by R. A. Furnanage and D. K. Wilson (U.S. Pat. No. 3,363,195 issued on Jan. 9, 1968). The stripe geometry reduces the threshold current for lasing (compared to broad area lasers) and limits the spatial width of the output beam. Since that early proposal, numerous laser configurations have been devised to implement the stripe geometry concept: (1) the oxide stripe laser; (2) the proton bombarded laser; (3) the mesa stripe laser; (4) the reverse-biased p-n junction isolation laser; (5) rib-waveguide lasers; and (6) buried heterostructures of various types.
The most commonly used configuration for the past eleven years, however, has been the proton bombarded, GaAs-A 1G aAs double heterostructure (DH) laser described, for example, by H. C. Casey, Jr. and M. B. Panish in Heterostructure Lasers, Part B, pp. 207-210, Academic Press, Inc., N.Y., N.Y. (1978). Despite its various shortcomings, lasers of this type have regularly exhibited lifetimes in excess of 100,000 hours, and a number have exceeded 1,000,000 hours (based no accelerated aging tests). Long lifetimes have also been observed in DH LEDs employing similar proton bombardment to delineate the current channel.
Several of the shortcomings of proton bombarded DH lasers are discussed by R. W. Dixon et al in The Bell System Technical Journal, Vol. 59, No. 6, pp. 975-985 (1980). They explored experimentally the optical nonlinearity (presence of "kinks" in the light-current (L-I) characteristics) and the threshold-current distribution of AlGaAs, proton-bombardment-delineated, stripe geometry DH lasers as a function of stripe width (5, 8, and 12 .mu.m) in cases in which the protons did and did not penetrate the active layer. They demonstrated that shallow proton bombardment with adequately narrow strips (e.g., 5 .mu.m) can result in satisfactory optical linearity (kinks are driven to non-obtrusive, high current levels) without the threshold penalty that has been associated with narrow-stripe lasers in which the protons penetrate the active layer. On the other hand, lasers with such narrow strips have exhibited a statistically meaningful, although not demonstrably fundamental, decrease in lifetime. In addition, failure of the protons to penetrate the active layer increases device capacitance and hence reduces speed of response and, moreover, increases lateral current spreading and hence increases spontaneous emission. In digital systems, the latter implies a higher modulation current to achieve a predetermined extinction ratio or a lower extinction ratio for a predetermined modulation current.
The concurrently filed application of R. W. Dixon et al, supra, describes stripe geometry, proton bombardment-delineated DH lasers in which satisfactorily high optical linearity, low capacitance, and low spontanenous emission levels are achieved by means of a current confinement scheme in which the current channel is narrower at the top near the p-side contact and wider at the bottom near the active layer. More generally, the Dixon et al application describes light emitting semiconductor devices (lasers or LEDs) having a semiconductor body, an active region within the body, and constraining means through which current flows from a major surface of the body to the active region, thereby causing radiative recombination of holes and electrons in the active region. The constraining means includes first means forming a relatively narrow first channel which extends from approximately the major surface into the body to a depth short of the active region, and second means forming a relatively wider second channel which extends from approximately that depth into or through the active region. Illustratively, the first and second means comprise high resistivity regions which bound the channel. These regions can be formed by a number of techniques including proton bombardment, oxygen bombardment, or suitable etching and regrowth of high resistivity material.